Snowden copied co-worker password, NSA tells Congress

June 9, 2013: This photo provided by The Guardian newspaper in London shows Edward Snowden. (AP)

The National Security Agency is telling Congress that former NSA analyst Edward Snowden gained access to at least some classified files by copying a password from a co-worker who has now resigned.

The unnamed civilian employee resigned last month after the government revoked his security clearance, according to a letter to the House Judiciary Committee.

Snowden has denied in interviews that he stole computer passwords or tricked some co-workers into giving him their passwords. But the NSA letter suggests that Snowden tricked at least one of them and copied his password without the co-worker's knowledge.

The head of U.S. intelligence, James Clapper, told senators this week that Snowden's access to so many classified files has accelerated plans to tighten clearance procedures and monitoring on government computers.